The present invention relates to films for use in heat-applied graphics, and more particularly to a urethane film or layer, preferably in a composite having a pigmented polyurethane graphic layer which exhibits a relatively higher melt temperature, and an adhesive layer which exhibits a relatively lower melt temperature.
U.S. Pat. No. 3,660,212 (Liebe) discloses a plastic athletic lettering material and associated method of manufacture, by which a flexible plastic sheet material can be cut to provide pigmented letters, designs, and the like for fusing to fabrics.
A liquid pigmented mass of heat curable plastic, for example plastisol, is knife-spread on a high gloss release sheet and heat cured, to serve as the outer, pigmented side of the sheet material to be formed. An unpigmented pre-formed flexible thermoplastic sheet, having a fusing temperature lower than the temperature of heat curing, is pressed to the newly cured, hot pigmented film, and fused to it. This thermoplastic serves as the inner adherent side of the material. Letters and designs cut from such material may be ironed onto fabric, utilizing heat sufficient to raise the inner, thermoplastic side to fusing temperature without affecting the outer, pigmented side.
Material manufactured in accordance with the technique of the '212 patent, exhibits good opacity, good color stability, and can, with some difficulty using multiple steps, be utilized to make a multi-color graphic, by adhering a first letter of one color, on top of a background of another color. One disadvantage of this is that the material curls during storage prior to use. This material has other disadvantages as well. The thickness is typically on the order of 10 mils, and the plastisol or similar vinyl material becomes embrittled, especially during cold weather and with the passage of time and repeated washing and drying cycles. As a result, the graphic does not have the same flexibility as the fabric, resulting in discomfort for the wearer and a low-quality appearance to the observer.
The technique disclosed in U.S. Pat. No. 4,423,106 (Mahn) overcomes several of the disadvantages of the technique described in the '212 patent. In the '106 patent, a laminated material is formed of a pigmented layer of polyurethane and a layer of amorphous polyester adhesive. The polyester adhesive layer is laminated to the layer of polyurethane under elevated and pressure. The temperature is high enough above the softening point of polyurethane to remove enough toluene solvent from the layer of polyurethane so that curling is prevented when the material is eventually cut into free form graphics and aligned on a preheated fabric. When a graphic of the laminated material is applied to fabric under pressure and heated sufficiently that the layer of polyester becomes adhesive, the laminated material adheres to the fabric.
Thus, the material manufactured in accordance with the '106 patent, provides some improvements relative to the material of the '212 patent, arising from the use of a polyurethane pigmented layer. These include aesthetics in the attached lettering, due in part to less thickness, (e.g., 6.5 mils), greater flexibility, and more tolerant ageing properties. The step of applying the graphics to the garment is also improved, as a result of the absence of curling. In some respects, however, the material of the '106 patent is inferior to that of the '212 patent. The opacity of the pigmented graphic is only fair, and color stability is likewise only fair. Adhesion of one color on top of another color, is relatively poor. Furthermore, the material prior to application on the garment, feels somewhat greasy.
A material made in accordance with the '106 patent is significantly more expensive to manufacture than a material made according to the '212 patent, but for many end uses, the trade off is favorable for obtaining the advantages associated with the polyurethane graphics layer of the '212 patent. Nevertheless, because of this cost differential, it is imperative that the material components used in practicing the techniques of the '106 patent, be as economical as possible. Therefore, urethane film utilized in accordance with the techniques of the '106 patent, has typically been manufactured by the so-called "blown film" process of extrusion.
In any extrusion process, particularly, the blown film type, some degree of lubrication is required to assure that the melted resin can flow along the extrusion dye surface without binding. According to well known techniques for continuous blowing of thermoplastic film, the melted material is subjected to a flow of compressed air, which expands the material in the nature of blowing up a balloon, thereby progressively decreasing the thickness to that of a film. To assure that air does not leak out of the "balloon", the thermoplastic material is confined through a neck or the like, which produces a seal to prevent loss of air pressure. The contact between opposed film surfaces at the neck, must be close enough to achieve the seal, without fusing the film surfaces together.
In the conventional production of polyurethane film, upon reaching the neck, the film has cooled but the material has not fully recrystallized. Therefore, the contact of these relatively soft film surfaces can only be made in the presence of a lubricant which acts as a release agent so the surfaces will not adhere. This lubricant is typically mixed into the polyurethane resin upstream of the blowing station, and is therefore an integral constituent of the melt. Approximately 0.5% of the melt mixture would be a lubricant such as Glycolube VL, an aramid wax, available from Lonza, Inc. This is an internal/external lubricant which has the disadvantage of blooming to the surface of the film.
The poor opacity results when a light color film is applied to a dark garment or when too much time, temperature, or pressure is used in applying the graphic to the fabric substrate. This poor opacity is due in part to a poor color density in the manufactured film based on technical limitations regarding how much pigment loading is possible before process limitations are reached. It is believed that such limits for presently commercialized blown urethane lettering film, are on the order of 11% for T.sub.i O.sub.2 in white film. The melt parameters of the presently known urethane products are such that the film softens and flows quite readily into the weave of the fabric when heated for application onto the fabric, thereby causing a loss of opacity and a major encapsulation of the garment fibers. This effectively results in a stiff hand of the applied graphic due to fabric encapsulation.
The presence of a substantial quantity of lubricant in conventional blown film, has been identified by the inventor of the present application, as giving rise to some of the significant disadvantages of heat applied graphic films commercialized based on the teachings of the '106 patent. The difficulty of multi-layer adhesion of one composite graphic film on top of another composite graphic film greatly limits the use of such composites for sport team applications. This deficiency has been known for many years, without a commercially available solution. This adhesion problem can be attributed to a general lack of compatibility between the polyester adhesive and the polyurethane film layer presently being used in practicing the '106 patent. Part of this incompatibility is due to the presence of the surface lubricant, which makes adhesion difficult.
Another problem resulting from surface contamination by the lubricant, limits use of the known composite material in connection with reverse image transfer systems, e.g., on a carrier sheet whereby a computer controlled blade cuts the desired graphics through the composite (urethane and adhesive), but not through the thick carrier. After cutting, the scrap film is peeled away and the graphic remaining in registry on the carrier with adhesive up, is sealed to the garment. After application of the graphic to the garment, the carrier is peeled away.
The surface characteristic of the lubricated film as used in the commercial implementation of the '106 patent, inhibit adhesion to the carrier, so that only large graphics can be cut, but with some difficulty. In particular, as the blade cuts a graphics pattern, the lack of adhesion to the carrier results in shifting of the urethane film during cutting. This prevents the cutting of clean corners, and can prevent completely cutting a corner. These drawbacks severely limit the commercial viability of the material manufactured in accordance with the '106 patent, with a carrier sheet.